Olefins polymers like polypropylene or polyethylene are widely used commercial polymers, whose success is based not only on the facts that it is possible to carry out the production at relatively low costs but also that the obtained materials meet the requirements with respect to good product properties and processability.
When the formed polyolefin particles are removed from the polymerization reactor, the discharged product is not pure polyolefin but contains portions of the medium, in which the polymerization took place. If the polymerization was carried out as gas-phase polymerization, the gaseous phase is partly concomitantly discharged as intergranular gas or as dissolved hydrocarbons. If the polymerization was a suspension polymerization, parts of the liquid suspension medium still adhere to the polyolefin particles after mechanical separation of liquid and solid phase and hydrocarbons are also dissolved in the polyolefin particles. For ecological, safety and quality reasons, these entrained parts of the polymerization medium have to be removed from the polyolefin particles because its components constitute an impact on the environment, gaseous hydrocarbons may result in the formation of explosive mixtures in downstream equipment and remaining non-polymerized components in the final polyolefin polymers may cause problems of quality such as odor formation. Furthermore, it is desirable to recycle unreacted monomer and comonomer to the polymerization process.
To remove the entrained parts of the polymerization medium from the polyolefin particles, it is therefore common practice to contact the particles with a stream of an inert gas, usually in countercurrent flow. Such a step is frequently denoted as “degassing” or “purging”. Often such a degassing or purging step is combined with a step of deactivating the polymerization catalyst and/or cocatalysts, e.g. by reacting the catalyst and/or cocatalysts with water.
For examples, EP 339 122 A1 discloses a two-step method for removing unpolymerized gaseous monomers from a solid olefin polymer while deactivating Ziegler-Natta catalysts and organometallic catalyst residues present in said solid olefin polymer, which is carried out in a single vessel. The solid olefin polymer is first countercurrently contacted with a first purge gas, preferably pure nitrogen, in an upper zone of the purge vessel, then transferred in the lower zone of the purge vessel and there countercurrently contacted with a second purge gas containing water, preferably pure nitrogen and steam.
U.S. Pat. No. 5,071,950 refers to a process for the continuous preparation of an ethylene/α-olefin copolymer in which the resulting ethylene copolymers are transferred to a let-down zone of reduced pressure and then the solid copolymer is freed of residual monomers and odor and flavor substances in a two-step way by first flushing with gaseous ethylene and then flushing with a mixture of nitrogen and steam. Similarly, EP 683 176 A1 describes a process for continuously manufacturing ethylene (co-)polymer in a gaseous phase in which the solid (co)polymer, after having passed a depressurization zone, is subjected to (1) a non-deactivating flushing with respect to the active catalytic residues, and subsequently (2) a deactivating flushing with a gaseous mixture of nitrogen, water and oxygen. Preferably, the gas for the non-deactivating flushing is the gaseous reaction mixture which circulates in the polymerization zone.
WO 2006/082007 A1 discloses an ethylene polymerization process in a gas-phase reactor in which the obtained polymer particles are discharged from the reactor, separated from the major part of the concomitantly discharged reactor gas and thereafter degassed, wherein the degassing is carried out with a propane fraction separated from the concomitantly discharged reactor gas.
WO 2008/015228 A2 describes a process to perform the finishing of polyolefins produced by gas-phase catalytic polymerization of one or more α-olefins in the presence of a polymerization diluent selected from a C3-C5 alkane, in which the polyolefin particles discharged from the gas-phase reactor are subjected to a first degassing step in which the polyolefin particles are countercurrently contacted with a gaseous stream containing at least 85 mol-% of a C3-C5 alkane and then to a second degassing step in which the polyolefin particles are countercurrently contacted with steam.
After having been degassed, the polyolefin particles are usually transferred to a melt mixing device such as an extruder or continuous mixer, in which the polyolefin particles, usually together with common additives, are melted, mixed and thereafter pelletized.
Furthermore, it is common practice that the polyolefin particles after having been degassed are first transferred to a storing unit such as a powder silo. This powder silo can serve as buffer if the work-up of the polyolefin particles is interrupted for a certain period of time such as for example if the cutting blades of the granulator of the melt mixing device need to be changed. The buffering allows, usually for several hours, continued operation of the polymerization reactor with stopping the polymerization. Otherwise a quite elaborated re-starting procedure would have to be carried out to resume the polymerization. Such a set-up however requires the installation of storage containers between degassing and pelletizing of the polyolefin particles and furthermore equipment for the pneumatic transfer of the particles from one vessel to the next has to be provided. Moreover, every additional pneumatic transport step results not only in additional investment costs but also in increased operational costs.
Thus, it was the object of the present invention to overcome the disadvantages of the prior art and to find a possibility for still having the flexibility to continue with the polymerization in the polymerization reactor for a certain time period if the melt mixing device is stopped, however without the necessity to install storage containers between degassing and pelletizing of the polyolefin particles and minimizing the requirement for pneumatic transport of the polyolefin particles.